In vivo molecular imaging makes it possible to measure the pharmacokinetic properties of an active ingredient in intact animals and humans. Molecular imaging encompasses nuclear techniques and optical techniques. Among nuclear techniques, position emission tomography (PET) and single photon emission computed tomography (SPECT) are the most sensitive techniques and the only ones to offer quantitative measurements.
The quantification of the measurements with PET or SPECT to study an active ingredient requires the intravenous injection of a radioactive tracer adapted to the study of said active ingredient. An image of the distribution of the tracer, and therefore the active ingredient, is then reconstructed from a temporal sequence of PET or SPECT images for measuring the concentration of the tracer.
The post-processing of the PET or SPECT image generally requires three steps to access the pharmacokinetic parameters. First, regions of interesting delimiting the pharmacokinetic organs, called pharmaco-organs, must be defined on the temporal sequence of PET or SPECT images, or on a morphological image recalibrated with the temporal sequence of PET or SPECT images. Then, the kinetics of the pharmaco-organs, i.e. the temporal activity curves of the tracer corresponding to the variations over time of the concentration of the tracer, must be extracted. Lastly, a physiological kinetic model can be defined, based on the kinetics and the concentration of the tracer in the plasma, making it possible to calculate the pharmacokinetic parameters of interest for the practitioner.
The extraction of the pharmacokinetic parameters through imaging using a tracer is therefore the final step providing access to information on the processing of the tracer by the body, thus able to provide information on the effectiveness of a drug or the evolution of a pathology.
Methods are known that make it possible to estimate the pharmacokinetic parameters and the blood concentration of the non-metabolized tracer as a function of time, called input function, from the extracted kinetics. Indeed, the input function is necessary to know the pharmacokinetic parameters, but it is not always directly visible in the image.
However, the pharmacokinetic parameters extracted using these methods suffer from a very strong bias when they do not benefit from additional information in the form of venous or arterial collection. Moreover, the existence of such a restriction makes these methods applicable only to a very small number of tracers.